This invention relates generally to devices for measuring linear travel and more particularly to devices for very accurately measuring the bidirectional movements of a conveyor.
For numerous applications in which materials are transported by conveyor from one processing station to another it is required that the advance of the conveyor be accurately measured so that the parameters of the material sensed at one station may be associated with the proper segment of the material at a subsequent processing station. For example, U.S. patent application Ser. No. 704,191 filed July 9, 1976, entitled "System and Method for the Measurement of Repetitive Patterns" by Josef K. Holy and assigned to the assignee of the subject application, deals with such a system used in the garment industry. In this just referenced system cloth is transported on a conveyor surface between a first station where the plaid characteristics of the material are measured to a subsequent station where computer control cutting devices automatically cut pattern pieces from the cloth goods. A good plaid match in the finish garment requires very accurate measurement of the travel of the conveyor so that proper correlation is maintained between the measuring and cutting operations.
For a number of reasons, prior art linear position measurement devices are inadequate for applications of the type just described. For example, in one such prior art device, which may generally be classified as an intermittent, reciprocating action, position measuring device, the distance measured is limited to the length of travel of a measuring head and movement of the conveyor has to be stopped to allow recycling of the head.
Prior art rotary type measuring devices are generally driven from the conveyor sprocket (bull gear), the sprocket drive shaft, or the rollers and are subject to errors caused by eccentricities, backlash in the gears and chains, and compliance between rotating conveyor members and the points on the conveyor where measurement is desired.
Prior art friction wheel type linear measuring devices are subject to errors caused by wear, slip and surface irregularities.